Method of improving chemical coating on aluminum



assignor to Kaiser Michael J. I Pryor, Spokane, Wash.,

Oakland, Calif.,

Aluminum & Chemical Corporation, a corporation of Delaware No Drawing. Application June 21, 1957 Serial No. 667,295

* Claims. 01. 14896.16)

This invention relates generally certain aluminum and aluminum alloy products. More particularly, the invention pertains to an improvedprocess for enhancing the corrosion resistance of chemically coated aluminum and aluminum alloy products of a nature such that entrapment corrosion presents a major problem.

Although the natural oxide film that forms on aluminum and aluminum surfaces affords protection to the metal under ordinary circumstances, severe climatic conditions, such as humid and salt-laden atmospheres, decreases this prot'ection. In order to enhance the corrosion characteristics of aluminum, a number of processes for producing asubstantiallyw integral chemical coat upon the aluminum surface by chemical changes in the surface of the metal have been developed. The chromate-phosphate-fiuoride type coatings are outstanding examples of chemical coatings for aluminum and aluminum alloys. Chemical coatings produced in solutions containing phosphate ions, chromate ions and fluoride ions as active coating producing ingredients has received wide spread usage for producing chemical coatings afiording good corrosion resistance to aluminum surfaces as well as serving as a satisfactory paint base coating. One such process of coating aluminum is commonly known in the trade as the Alodine process. However, it has been found that with the chromate-phosphate-fluoride coatings the corrosion resistance of the metal, particularly to marine atmospheres, is impaired when the product is of such a nature that solid deposits may become easily entrapped. This corrosion is attributed to the presence of non-uni form concentrations of residual hexavalent chromium ions in the chromate-phosphate-fluoride film. These non-uniform hexavalent chromium concentrations may be present either initially or may develop subsequently during service in corrosive atmospheres. The differences in concentration of soluble oxidizing ions in adjacent regions within the coating will lead to the establishment of aggressive local corrosion cells. The weak points in the coating where more effective leaching by the action of rain takes place would contain lower concentrations of oxidizing ions and therefore would be particularly vulnerable to a rapid and highly selective pitting corrosion.

Therefore, it is primary object of this invention to provide an improved chemical coating process for aluminum and aluminum alloy products, of a nature such that entrapment corrosion presents a major problem, wherein the chemical coating affords greater corrosion protection.

It is a further object of this invention to substantially reduce or eliminate the differences in concentrations of soluble oxidizing ions in chromate-phosphate-fluoride chemical coatings.

It is also an object of this invention to provide an improved chemical coating process for aluminum and aluminum alloy products, of a nature such that entrapment corrosion presents a major problem, wherein the concentration variations of soluble hexavalent chromium ions to the art of coating mersedin a tank containing within the coating are reduced or substantially eliminated.

These and other objects and advantages of the invention will be apparent from the ensuing detailed description of the invention.

The invention is based upon the discovery that treatment of chromate-phosphate-fluoride coatings produced on aluminum and aluminum alloy products, wherein soluble hexavalent chromium ions may be present Within said coating, with a solution containing reducing ions, that is, a final reducing rinse, will reduce or substantially eliminate the presence of these hexavalent chromium ions. The reducing rinse should contain a powerful reducing agent that does not give insoluble oxidation products and lies within the pH range of about 3.0 to 7.0. An example of a preferred reducing agent is a sodium bisulfite solution containing from about 15 to ounces of sodium bisulfite per 100 gallons of water, however, other reducing agents are contemplated within the scope of the invention. While the above ranges of sodium bisulfite concentration are preferred, concentrations as low as one ounce of sodium bisulfite per high as 300 ounces of sodium Water may be used to reduce residual hexavalent chromi um ions. The aluminum product after being coated with the chromatephosphate-fluoride coating may be imthe sodium bisulfite solution, or it may be. spray rinsed with the sodium bisulfite solution. In the case of spray rinsing a mixing valve may be used whereby concentrated sodium bisulfite is mixed in the water sprays in the desired concentration. The operating temperature of the sodium bisulfite solution is not critical. After treatment with the sodium bisulfite solution, the coated surfaces are then dried.

In the production of chromate-phosphate-fiuoride coatings on aluminum and aluminum alloy products the aluminum surface is treated with an acid aqueous solution containing dichromate ions, phosphate ions, floride ions and hydrogen ions. The coating solution may be maintained at any temperature from ordinary room temperature to F. or more, the action of the solution being accelerated by heat. The usual time of treatment to produce the coating ranges from 1 to 2 minutes to 5 to 10 minutes, depending upon the temperature of the solution. Heretofore, it has been the usual practice to rinse the coated surfaces with a dilute chromic acid solution containing from /2 to 8 ounces of chromic acid per 100 gallons of water, after which the surfaces are dried. As taught by the instant invention, substituting the oxidizing rinse of dilute chromic acid solution with a reducing rinse, e.g. a dilute solution of sodium bisulfite, the presence of oxidizing ions within the coating is reduced or substantially eliminated. The following description of tests comparing the corrosion resistance afiorded by chromate-phosphatefluoride coatings With a chromic acid rinse and with a sodium bisulfite rinse in a corrosive marine environment establishes the elficiency of the sodium bisulfite rinse in reducing corrosion.

Samples of a louvered screen of the type known as Shade Screen, 12 inches square fabricated from 0.008 inch thick 5052 aluminum alloy (containing 2.5% magnesium as a main alloying element) were coated with a chromate-phosphate-fluoride coating in the conventional manner. Half of the samples were rinsed in dilute chromic acid according to conventional practice before drying. The remaining samples were rinsed in a room temperature solution containing 133 ounces of sodium bisulfite in 100 gallons of water. The samples were tested by exposure to a corrosive marine atmosphere at Daytona Beach, Florida. The samples were partly sheltered by an 100 gallons of water and as bisulfite per 100 gallons of.

suitable commercial '2 o: overhang to accentuate the pickup of wind blown dcposits.

After three months the samples rinsed with chromic acid showed perforations of the ribs and louvers, while tliose samples rinsed in sodium-bisulfite showed'no'perforations. After nine months eitposure, the samples rinsed in chromic acid showed an average of 25 broken ribs and louvers per sample, whereas those rin's'edxwitli sodum-bisulfite showed no broken ribs or louvers. These tests under closely simulated service conditions clearly demonstrate the marked superiority of a reducing rinse for products such as louvered aluminum screen.

Although the invention is shown specifically in the" above example as applying to the chromate-phosphatefluoride type of chemical coating, it is not to be" con-' strued that the invention is necessarily limited thereto. The invention applies to any chemical coating for aluminum and aluminum alloy products wherein soluble strong- 1y oxidizing ions may be incorporated in the' coating in wherein these non-uniform concentrations may be responsible for aggrwsive local a non-uniform manner and cell corrosion.

Various modifications of processing conditions and'in solution compositions permitting the'substantial realization of the results herein set forth are deemed within the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. In a'process for chemically coating aluminum and aluminum alloy products, of a nature such thatehtrap= ment corrosion presents a major problem, wherein said' products are subjected to the action of an acid aqueous solution containing chromate ions, phosphate ions and solution containing chromateio'ns, phosphate ions and" fluorine ions as active coat-forming constituents to' produce a coating on said surface, the improvement cornprising the'step of treatingsaid coating with a dilute rinsing solution of sodium bisulfite;

3. A process accordingto claim 2 wherein said sodium bisulfite solution'lies within the pH range of from about 3.0 to 7.0.

4. A"pi'ocess according to'claini'3 wherein the concentration of sodium bisulfite in said rinsing-solution ranges" from'l to 300 ounces of sodium'bisulfite per 100"ga1lons'" of water.

5. A process according to claim 3'wherein the conc'enfrom 15 to 150 ounces of sodium bisulfite per IOO gaI- loris of Water.

References Cited in the: file or this mm UNITED STATES'P'ATENTS 2,737,498 Frasch Mar. 6, a 195.6; 2,768,104 Schuster et al. Oct. 23,1956 2,777,785 Schuster et a1. Jan; 15; 1957 in said rinsing soluti'oti ranges 

1. IN A PROCESS FOR CHEMICALLY COATING ALUMINUM AND ALUMINUM ALLOY PRODUCTS, OF A NATURE SUCH THAT ENTRAPMENT CORROSION PRESENTS A MAJOR PROBLEM, WHEREIN SAID PRODUCTS ARE SUBJECTED TO THE ACTION OF AN ACID AQUEOUS SOLUTION CONTAINING CHROMATE IONS, PHOSPHATE IONS AND FLUORINE IONS AS ACTIVE COAT-FORMING CONSTITUENTS TO PRODUCE A COATING ON SAID SURFACES, THE IMPROVEMENT COMPRISING THE STEP OF TREATING SAID COATING WITH A RINSING SOLUTION CONTAINING A REDUCING AGENT THAT DOES NOT GIVE INSOLUBLE OXIDATION PRODUCTS, SAID SOLUTION HAVING A PH OF ABOUT 3.0 TO 7.0. 